1. Field of the Invention
The present invention generally relates to liquid level measuring systems and, more particularly, to a bubble type fuel measuring system having specific application for measuring the quantity of fuel in tanks of railroad locomotives. A particular feature of the fuel measuring system according to the invention is that it can be used on a variety of locomotives made by different manufacturers.
2. Description of the Prior Art
There has been a continuing need in the railroad industry to develop an accurate fuel gauge. Over the years, there have been several attempts at gauging the amount of liquid in a tank. Early precursors of present day gauges used sight glasses affixed to the side of the tank which would fill with liquid to a level of the tank. A reading would then be taken correlating height of the liquid in the sight glass tube to gallons of liquid in the tank. The natural problems with this were sight glass damage or dirt contamination which caused an inability to read the gauge. Secondly, one had to be at eye level to the gauge to read it.
The second method employed was a mechanical type gauge which employed a float arrangement to move a needle, typically through a gear arrangement. As the liquid was expended, the float would move lower in the tank and therefore display less fuel. Problems with this design were that it would not withstand the effects of a "sloshing liquid" and the gear assembly or float/arm would become bent or broken. Secondly, irregular cross elevation of the track structure would cause the device to read inaccurately. Third, one also had to be at gauge level to read the device.
The third method of measuring liquid is a rather old idea and that is to pass a hollow tube vertically down through the tank and then pass air through the tube. Depending on the pressure needed to force air through the tube, a manometer type of gauge was hooked in parallel with the air source and vertical tube and the effective pressure was read with the use of a cross index scale on the gauge to read volume in the tank.
The fourth method of measurement employed is a pressure diaphragm that resides in the bottom of the tank itself and measures the pressure of the liquid. This pressure measurement is converted to volume or gallons.
The fifth method of measurement that has been employed is a capacitance type of system whereby a large open frame capacitor is placed inside the fuel tank. The liquid acts as a variable dielectric slug, with more or less capacitance according to the level of the liquid. The capacitor is placed in a bridge network and the current generated by the imbalance of the bridge is converted to a measure of volume, pounds or gallons.
Of these five systems, only the first four have been employed by the railroad industry. The first two while accurate have both logistic and physical problems; i.e., they are impossible to read from the locomotive cab and tend to be contaminated by dirt making reading difficult. The fifth method is employed extensively in the aircraft industry.
As mentioned, the third or bubble type liquid level measuring systems have been known for some time. In these systems, air under pressure is bubbled through the liquid in a tank. Depending on the amount of liquid in the tank, the geometry of the tank, the temperature of the liquid, among other factors, the pressure of the air bubbled through the tank will vary. For a particular application, all variables other than the quantity of the liquid must be held constant or some compensation provided so that a measurement of pressure will provide an accurate indication of the quantity of the liquid in the tank. Examples of bubble type liquid level measuring systems are disclosed in U.S. Pat. No. 3,262,313 to Hanna and U.S. Pat. No. 4,409,833 to Thomson et al. Neither of these patents are directed to a liquid level measuring system having specific application to railroad equipment.
U.S. Pat. No. 4,454,759 to Pirkle discloses a fuel gauge for locomotives which may be generally characterized as of the pressure transducer type, that is, the fourth type described above. U.S. Pat. No. 1,946,175 to Murphy et al. discloses a water level indicator in a railroad tank car. Like the Pirkle fuel gauge, the Murphy et al. indicator is of the pressure transducer type.
Other bubble type gauges are known in the prior art, but these are designed to measure other properties of a liquid rather than the level or quantity of the liquid. For example, U.S. Pat. No. 1,822,458 to Rowland et al. discloses a bubble type gauge which is designed to measure the density of the liquid.
What is needed in the railroad industry is a device that contains the minimum amount of retrofitting of the fuel tanks to operate. Any system that employs the diaphragm or pressure transducer technology, i.e., the fourth method described above, or the capacitor technique, i.e., the fifth method described above, necessarily must have a portion of the package inside the tank. This means a method for placing it in the tank and the problems inherent in equipment failures and repair in that medium. There are of course other technologies which could be employed such as, for example, sonar or radar, but these would suffer much the same problems as far as retrofit requirements and industry acceptance.